The invention relates generally to a combustion system, and more particularly, to a fuel-flexible combustion system and method of operation.
Various types of combustors are known and are in use in systems such as in combined cycle power plants. Typically, the combustors for such systems are designed to minimize emissions such as NOx and carbon monoxide emissions. In most natural gas fired systems, the combustors are operated using lean premixed flames. In these systems fuel is mixed with air upstream of the reaction zone for creating a premixed flame at lean conditions to reduce emissions from the combustion system. Unfortunately, the window of operability is very small for such combustion systems. Further, it is desirable to avoid combustion dynamics while keeping NOx low and avoiding lean blow out of the flame. Designs are typically targeted for a narrow fuel composition range, thereby making a system designed for natural gas incompatible with a system designed to use gasified coal or synthesis gas fuel.
Certain other systems employ diffusion combustion to minimize emissions through diluent augmentation in the reaction zone. For example, in an integrated coal gasification combined cycle (IGCC) system, steam or nitrogen may be employed as a diluent to facilitate the combustion and reduce emissions from the combustor. Typically, for an IGCC system, the combustor is designed to operate in a diffusion mode using a coal gasified fuel and may have a backup firing mode using natural gas in a diffusion mode. However, it is challenging to design a combustor that can operate on coal gasified fuels having varying calorific heating values while maintaining low emissions. The current IGCC combustors employ diffusion combustion and are designed on a site-by-site basis according to the gasified fuel stock. This results in specific combustion systems that have limited fuel flexibility in order to meet emission requirements.
Accordingly, there is a need for a combustion system that will work on a variety of fuels while maintaining reduced emissions. It would also be advantageous to provide a combustion system that has sustained low emission firing with a backup fuel and is adaptable to different power plant configurations while maintaining the overall power plant efficiency.